1. Field of the Invention
The present invention relates to high-speed, high precision measurement of the distance between two surfaces, one of which is on a substantially transparent element. In particular, the invention relates to an apparatus and method for measuring and compensating birefringence in the transparent element.
2. Description of the Prior Art
A frequently-encountered problem in industrial inspection and quality control is the precise measurement of small distances between surfaces. In magnetic data storage systems, for example, it is required to measure the flying height of a slider on a rapidly rotating rigid disk in order to verify the performance of the slider assembly. The flying height, as used herein, is the distance between the magnetic head pole and the surface of the rotating rigid disk; see, e.g., M. F. Garnier, et. al., U.S. Pat. No. 3,855,625 issued Dec. 17, 1974. The flying heights are generally less than 250 nm (10.mu.-inch) depending on the design of the slider, and may be as close as a few tens of nanometers. In that the flying height is a critical performance parameter in magnetic data storage systems, the dynamic measurement of flying height plays a central role in the design and production testing of sliders.
Optical means for measuring the flying height are known as optical flying height testers (OFHT's). High-precision OFHT's are almost invariably based on interferometry, Interferometers are capable of determining the distance to an object, the topography of the object, or like physical parameters involving physical lengths (see, for example, Chapter 1 of the book Optical Shop Testing, second edition, edited by Daniel Malacara (Wiley, New York, 1992). One of the fundamental difficulties of prior art optical techniques is that the interface between the slider ABS and a real hard disk cannot be inspected directly. Most OFHT's therefore use a transparent surrogate disk, most commonly a glass disk, in place of a real magnetic hard disk. The measurement light beam passes through the glass disk before and after interacting with the slider-glass interface. The optical interference effect at the slider-glass interface provides the necessary information for flying height measurement. Examples of prior-art OFHT's incorporating a glass disk may be found in U.S. Pat. No. 4,593,368 to D. A. Fridge, et al. and in U.S. Pat. No. 5,280,340 to C. Lacey. Further examples of prior art systems for measuring the flying height of a slider assembly are disclosed in B. Bhushan, Tribology and Mechanics of Magnetic Storage Devices, pp. 765-797 (New York: Springer-Verlag, 1990).
The interaction between the glass disk and the measurement beam is important to the accuracy of the flying-height measurement. This is particularly true for a class of prior-art OFHT's that employ polarized light as part of the measurement principle. A polarization-based OFHT measures the flying height by the detection and analysis of the polarization-dependent characteristics of a beam or combination of beams reflected at an oblique angle from the slider-disk interface. Advantageous features of polarization-based OFHT's include high accuracy, improved reliability with respect to competitive systems, and the ability to measure down to actual contact. Examples of prior art polarization-based OFHT's are provided in commonly-owned U.S. Pat. Nos. 4,606,638 and 5,218,424, both to G. Sommargren, my commonly owned copending U.S. Patent Application entitled "Interferometer and Method for Measuring the Distance of an Object Surface with Respect to the Surface of a Rotating Disk", filed Jan. 31, 1995 and bearing U.S. Ser. No. 08/381232, and in my commonly owned copending U.S. Patent Application entitled "Optical Gap Measuring Apparatus and Method", filed Mar. 22, 1995, and bearing U.S. Ser. No. 08/408907.
A difficulty with polarization-based OFHT's is that they are sensitive to any polarization-dependent phenomena in the glass disk, including birefringence, which may be defined as polarization-dependent variation in the index of refraction of the glass disk. Birefringence modifies the polarization state of the measurement beam in a manner inconsistent with the measurement principles as taught in the above-cited prior art. Of special concern is the influence of birefringence generated by the stress pattern in a rapidly-rotating glass disk. The resulting measurement errors can be as large as 50 nm. Although polarization-based OFHT's are well known in the art, the prior art does not provide any means of measuring or compensating birefringence in such polarization-based OFHT's.
An additional deficiency in prior art methods of flying height testing is the phase change that occurs at the slider surface upon reflection. The phase change can easily be misinterpreted as a change in flying height, resulting in errors as large as 20 nm. To correct for this effect, one must know the phase change exactly, using a priori knowledge of the complex index of refraction of the material. See for example, the article entitled "lnterferometric measurement of disk/slider spacing: The effect of phase shift on reflection," by C. Lacey, R. Shelor, A. Cormier (IEEE Transaction on Magnetics). Most often, an instrument known in the art as an ellipsometer measures the complex index of the slider in a separate metrology step, independent of the OFHT. In my commonly owned copending U.S. Patent Application entitled "Optical Gap Measuring Apparatus and Method", filed Mar. 22, 1995, and bearing U.S. Ser. No. 08/408907, a method is proposed for measuring the complex index of the slider in situ, comprising a polarization-based OFHT and analysis of data acquired during a load or unload of the slider. This approach obviates the need for a separate metrology station, since the apparatus for the complex-index measurement comprises substantially the same apparatus employed for the flying height measurement. However, since the method and means disclosed in the aforementioned copending U.S. patent application Ser. No. 08/1408907 also comprise the analysis of a polarized beam that passes through the glass disk, birefringence can adversely affect the accuracy of the complex index measurement.
An alternative approach to measuring the complex index of refraction of the slider is to incorporate a known form of ellipsometer in an existing flying height tester. Known forms of ellipsometer are taught in Chapter 27 of the book "Handbook of Optics", vol. II (McGraw-Hill, Inc., 1995), pp.27.1-27.27. However, since ellipsometers analyze the change in polarization of a beam reflected at an oblique from the surface of the material being tested, birefringence in the rotating glass disk also adversely affects the ellipsometric analysis. The birefringence phenomenon therefore places severe limitations on the ability of an ellipsometer to perform in-situ measurements of the complex index. Thus the prior art does not provide a satisfactory method and means for in-situ measurement of the complex index in the presence of glass-disk birefringence. These disadvantages of the prior art are overcome by the present invention.
There is therefore a critical, unmet need for a method and apparatus for measuring and compensating birefringence in rotating disks, particularly with regard to flying height testing and related techniques for in-situ measurement of the index of refraction of sliders. These disadvantages of the prior art are overcome by the present invention.